Manufacture of cellulose triacetate textile products



2,993,040 MANUFACTURE OF CELLULOSE TRIACETATE TEXTILE PRODUCTS Denis William Groomhridge and Reginald Henry John Riley, Spondon, near Derby, England, and James Hague learson, Thur-so, Caithness, Scotland, assignors to Britlsh Celanese Limited, a company of Great Britain No Drawing. Filed Aug. 21, 1957, Ser. No. 679,516

Claims priority, application Great Britain Aug. 23, 1956 8 Claims. (Cl. 260-230) This invention relates to the manufacture of cellulose triacetate filamentary materials by wet spinning from acetic acid solutions.

The best known method of making cellulose acetate givesas the immediate product a solution of the cellulose acetate in acetic acid, which in the present specification will be referred to simply as the acetylation solution. For many years the cellulose acetate, while still in the acetylation solution, was always subjected to a ripening operation before being precipitated, with the aim of converting it into the acetone-soluble type having an acetyl value (reckoned as acetic acid) about 52.5 56%. Quite recently a type of cellulose acetate of acetyl value above 59%, and especially above 60%, has attracted attention as the basis of textile materials having new and very desirable properties; this type, known as cellulose triacetate, is obtained by greatly curtailing or even omitting the ripening operation.

- I ted States Patent Cellulose triacetate can be formed into filaments by dry spinning and wet spinning methods, the latter being useful particularly for making high denier products such as tow for cutting into fibres of staple length. An excellent spinning solvent for this purpose is methylene chloride in admixture with a little methanol. To make a methylene chloride solution of cellulose triacetate which has been obtained in solution in acetic acid involves precipitating the cellulose triacetate from the acetylation solution, washing it free from acetic acid and then redissolving it in the methylene chloride. It would clearly be more economical if the cellulose triacetate could be spun directly from the acetylation solution, and various proposals for doing this will be found in the literature of the subject. In particular it is known that acetylation solutions of cellulose triacetate in acetic acid can be wet spun using a dilute aqueous acetic acid as the coagulant, and good methods of doing this are described in U.S. application S. No. 642,756 filed February 27, 1957, and United States application Serial No. 679,471, filed August 21, 1957.

However, when acetylation solutions, or for that matter other solutions, of cellulose triacetate in acetic acid are spun, certain practical difficulties are met with which do not arise with solutions in methylene chloride/methanol mixtures, and which are ascribed to the fact that, of the two solvents, acetic acid is inherently much the less powerful. Thus even at cellulose triacetate concentrations near the minimum desirable on other grounds for spinning, the solutions in acetic acid are frequently so viscous that they require the use of inconveniently high pressures to force them through the filters; moreover the filters often clog rapidly, so still further increasing the pressure needed to maintain a constant How of the solution to the spinning pumps and jets.

It is an object of the present invention to improve the spinning properties of acetic acid solutions of cellulose triacetate, and in particular to reduce their viscosity and. improve their filtering properties.

According to the invention a solution of cellulose triacetate in acetic acid is heated to a temperature of at least 70 C. for a time which is at least equal to the minimum useful period as defined below, but which is insufficient to cause the acetyl value of the cellulose tri- 7 acetate to fall below 59%, or better 60%. In the most important application of the invention the solution treated is an acetylation solution in which the acetylation catalyst, which will usually be sulphuric acid, has been neutralised. v

The neutralisation of the catalyst may be effected in a known way, as by adding to the acetylation solution a neutralising compound such for example as a carbonate or acetate of a metal of group I or II of the periodic table, e.g. a carbonate or acetate of sodium, calcium or magnesium, the amount added being at least equivalent to the amount of catalyst employed and preferably in excess up to 20-30%. Magnesium compounds are particularly useful for this purpose. In addition the concentration of cellulose triacetate in the solution may need to be adjusted, e.g. by adding further acetic acid, in order to bring the viscosity of the solution (as determined after the heat-treatment of the invention) to a level suitable for spinning.

The textile properties of the materials produced are often best when the solution contains 1-3%, and especially l.25-2%, of its weight of water, and if as is usual the Water content of the acetylation solution is below this, further water may be added.

The minimum useful period for which the solution is to be heated decreases with increasing temperature in a linear manner and is 180 minutes at 70 C. and 10 minutes at 120 C. It is however usually best to extend the period of heating considerably beyond the minimum useful period, partly because a greater effect will usually be attained by so doing, partly because inherent differences between different solutions are thereby reduced, and (especially at the higher temperatures) partly for the practical reason that the heating of large masses of viscous material for very short periods involves technical ditficulties. The upper limit tothe heating period depends not only on the temperature, but also (at the higher temperatures) on the composition, in particular the watercontent of the solution. Thus at temperatures up to about C. and water-contents up to about 3%, the upper limit is very high, e.g. 7 days or more at 80-85 C., and is therefore of little practical importance. At higher temperatures up to about C. but with watercontents below about 1% and especially below 0.85%, the upper limit is still very high, so that again there is in practice little or no danger of exceeding it. With higher water-contents the upper time limit at the higher temperatures is however much shorter; for example with awater-content of about 1.252% a temperature of about 100-110 C. is preferably not maintained for more than 6 hours. Periods of heating which, having regard both to the initial viscosity of the cellulose triacetate and the properties desired in the product, cause a degree of degradation (loss in average molecular weight) of the cellulose triacetate which in any particular case is regarded as excessive will naturally be avoided.

It is advantageous to mix or stir the solution continuously during atleast part of the heat treatment. For example it may be heated in a vessel provided with a stirrer or in a mixer 'of a known type, or it may be circulated round a system in part at least of which it is heated. If desired more than one of these expedients may be used- On the other hand when temperatures 7 above about -115 C. are to be used, and the period of heating kept short, the solution may with advantage be heated in the form of a thin flowing film, especially as a film flowing down a suitably heated vertical or inclined surface. r

Thesolution need not be at the same temperature duringjthe whole of thetreatment. Indeed it is advantageous -when.the solution is held at about 70-909C.

tor a long periodtoheat it more strongly, eg; to 100 A 3 115 C. or higher, before the main filtration for a short period such that neither the acetyl value nor the viscosity of the cellulose triacetate is unduly reduced. Thus when a solution is being transferred, e.g. by gravity feed or pumping, from an acetylation plant to the spinning positions, it is preferably kept at a temperature of 70-90 C. and particularly 80-85 C. throughout the whole or nearly the whole of the time needed for the transfer, except for a short period before the main filtration when it is heated to lll5 C. or above. When the solution is subjected to this stronger heating in bulk, and not as a thin flowing film, it is preferably thoroughly stirred or otherwise subjected to a strong shearing action.

In a preferred method of putting the invention into practice there is employed an acetic acid solution of cellulose triacetate containing about 11-13% by weight of the cellulose triacetate and 13%, and especially 1.25- 2%, by weight of water. As soon as possible after its formation, this solution is heated to about 80 85 C. Thus when an acetylation solution is being used, it is preferably heated as soon as possible after the neutralisation of the catalyst. It is then forwarded at about this temperature to the main filters, and from them to the spinning positions. Advantageously the solution is heated to l00-l15 C. or, if suitable equipment is available, to a higher temperature near or above the boiling point of acetic acid, say to l20-l30 C., for a short period preceding the main filtration, though not of course for long enough to cause any undue hydrolysis or degradation of the cellulose triacetate; usually between about 2 minutes and 3 hours, according to the temperature, will be found to be satisfactory. During this period of stronger heating it is particularly advisable that the solutron be either thoroughly stirred or otherwise subjected to a strong shearing action or be caused to flow as a thin film, as already described.

A solution which has been given the treatment described may be filtered and then used in the manufacture of yarns or tows or other filamentary materials by wet spinning, preferably by a process described in US. application S. No. 642,756 filed February 27, 1957, or especially United States application Serial No. 679,471. Thus it may be extruded into a bath of aqueous acetic acid as coagulant, and the filaments so formed stretched before they have become completely hardened. Advantageously such stretching is effected while the filaments are in contact with a liquid having a higher softening or swelling action on cellulose triacetate than has the coagulant, preferably an aqueous acetic acid of higher concentration and/or temperature than the coagulant, as described in United States application Serial No. 679,471. For example, while a suitable coagulant is aqueous acetioacid of concentration 5-25% and especially about 12-20%, at a temperature such that its swelling action on cellulose triacetate does not substantially exceed that of an 18% aqueous acetic acid at 18 C., the stretch-assisting liquid may be an aqueous acetic acid of concentration about 35-55% at about l525 C., or an aqueous acetic acid of concentration about l620%, conveniently the same as that of the coagulant, at about 7090 C., or an acetic acid of intermediate concentration at an intermediate temperature.

Another useful method consists in extruding the solution into a coagulant bath through which the filaments pass for a distance only sulficient, or not much more than sufficient, to give them coherence, and in any case much too small to allow them to attain equilibrium with the coagulant as regards acetic acid content. In many cases it is sufiicient to pass the filaments through the bath for about 1-4 inches, though immersion over somewhat longer distances, e.g. 4-8 inches, may be necessary when spinning coarse filaments or when spinning at particularly high speeds. If the vessel containing the coagulant is made very small, e.g. with a cross-sectional area little larger than is'nee'ded to contain the spinning jet, it is possible without coming into contact.

to supply the coagulant in the form of water or a highly dilute acetic acid, for example such as results from the washing of the stretched filaments, the acetic acid needed to give the desired concentration being then derived wholly or in part from the spinning solution. The filaments on leaving the bath are allowed to carry with them a substantial amount of coagulant, by means of which they become set to a greater extent as they travel through the air. This involves the transfer of acetic acid from the filaments to the liquid carried along by them, which thus becomes increasingly concentrated. It is advantageous to allow the process to continue until an approximate equilibrium as regards acetic acid content is established between the filaments and the liquid. The concentration of acetic acid at which this equilibrium is established can be readily controlled by means of an adjustable wiping or equivalent device, e.g. an adjustable yarn guide or adjustable squeeze rolls, positioned near the point at which the filaments leave the coagulant bath, and adapted to wipe off or squeeze out part of the coagulant which the filaments carry with them from the bath, and if desired to return it to the vicinity of the spinning jet. This adjustable wiping or equivalent device is preferably set so that approximate equilibrium between the filaments and the liquid is established at an acetic acid concentration between about 40% and 55%. When or shortly before this equilibrium has been established, the filaments are in a suitable condition for stretching in accordance with the invention, and may be passed to a stretching device by means of which they can be stretched without applying to them any further stretch-assisting agent.

The device employed for stretching the filaments may be of the well known type involving two or more rollers running at different peripheral speeds, the filaments making one or more turns first round the slower running and then round the faster running roller or rollers. The rollers may be provided with skew idler rollers or other means for enabling a number of laps to pass round them When the filaments are to be treated with further stretch-assisting agent, one or both rollers, or the filaments during their passage between them, may dip into a bath thereof.

The invention is particularly useful in the manufacture of tows containing large numbers of individual filaments, e.g. 1000 filaments or more, such as are employed in the production of staple fibres. It may however also be applied with advantage to the manufacture of other types of filamentary material, for example yarns of low or moderate denier and also monofils, e.g. thick filaments useful in making bristles and the like, artificial straw, etc., such materials being included within the term filaments as it is used in this specification. e

The improvement in the filtering properties of acetic acid solutions of cellulose triacetate which can be obtained by the application of the invention goes very far beyond what could be regarded as a normal reduction of viscosity caused by rise in temperature. The following example illustrates both the extent of this improvement, and also the unexpectedly good tensile. properties which have been observed in filamentary materials made from the treated solutions by wet spinning.

Example A solution of a cellulose triacetate of acetyl value 61.07% in acetic acid, having a cellulose triacetate concentration of 12.5% and a water content of l.7 5% by weight, obtained by the acetylation of cellulose in the presence of acetic acid followed by neutralisatiou of the sulphuric acid used as the acetylation catalyst and adjust rnent of the concentration and water content, was heated while in transit to the main filters of a spinning installation to. a temperature of -85 0., being maintained at this temperature for some 36 hours before being filtered. Afttr filtration the solution was forwarded to the spinning positions, and extruded at a temperature of about C. through a spinning jet having 10,578 holes each of diameter 0.05 mm. into aqueous acetic acid of concentration 18% at a temperature of 18 C. Before the filaments formed had been freed from adherent coaguiant they were stretched by about 12% of their length while softened by an aqueous acetic acid of concentration about 30% at a temperature of C., after which they were washed and dried while substantially free from tension. A tow having a tenacity of 1.27 grams per denier and extensibility 28.7% was obtained.

When the process was modified by heating the solution to 6065 C. instead of 80-85 C., not only was a much higher pressure needed to force the solution through the filter at a reasonable rate, but also the filter became blocked and had to be renewed after passing only one third of the amount of solution which could be filtered without changing the filters when the higher treatment temperature had been used.

The filtration properties of the solution could moreover be still further improved by heating it to 115 C. while subjecting it to a rapid shearing action for about 2 hours before filtering it.

Having described our invention, what we desire to secure by Letters Patent is:

1. Process for improving the filtering characteristics of a solution of cellulose triacetate in acetic acid, in which solution the acetic acid is the sole acid constituent and water is present in an amount of up to 3% by weight, which comprises heating the solution to a temperature of C. to 120 C. for a minimum time which decreases in a linear manner as the temperature increases and ranges between 180 minutes at 70 C. and 10 minutes at 120 C., the time of heating being insufficient to cause the acetyl value of the cellulose triacetate to fall below 59%.

2. In a process wherein cellulose is acetylated to cellulose triacetate in a reactant mixture containing acetic anhydride and wherein the resultant acetylation solution, after any unreacted acetic anhydride therein is destroyed,

is subjected to filtration and spinning, the improvement which comprises heating said acetylation solution prior to said filtration step to a temperature of 70 C. to 120 C. for a minimum time which decreases in a linear man ner as the temperature increases and ranges between 180 minutes at 70 C. and 10 minutes at 120 C., the time of heating being insuflicientto cause the 'acetyl value of the cellulose triacetate to fall below 59%.

3. Process according to claim 1, wherein the cellulose triacetate solution has been obtained by the acetylation of cellulose followed by neutralisation of the acetylation catalyst.

4. Process according to claim 1, wherein the cellulose triacetate solution contains 13% by weight of water.

5. Process according to claim 4, wherein the cellulose triacetate solution is heated for at least 12 hours to a temperature between and C.

6. Process according to claim 4, wherein the cellulose triacetate solution is heated above C. for at most 6 hours.

7. Process according to claim 5, wherein the solution is also heated to a temperature above 100 C. for a period less than 6 hours while it is submitted to a vigorous shearing action.

8. Process for improving the filtering properties of a solution of cellulose triacetate in acetic acid, in which solution the acetic acid is the sole acid constituent and water is present to the extent of 1 to 3% by weight, which comprises heating the solution to a temperature of about 80 to 85 C. for about 36 hours.

References Cited in the file of this patent UNITED STATES PATENTS 2,456,688 Dreyfuss et al. Dec. 21, 1948 2,768,870 Drisch Oct. 30, 1956 FOREIGN PATENTS 720,030 Great Britain Dec. 15, 1954 

1. PROCESS FOR IMPROVING THE FILTERING CHARACTERISTICS OF A SOLUTION OF CELLULOSE TRIACETATE IN ACETIC ACID, IN WHICH SOLUTION THE ACETIC ACID IS THE SOLE ACID CONSTITUENT AND WATER IS PRESENT IN AN AMOUNT OF UP TO 3% BY WEIGHT, WHICH COMPRISES HEATING THE SOLUTION TO A TEMPERATURE OF 70%C. TO 120*C. FOR A MININUM TIME WHICH DECREASES IN A LINEAR MANNER AS THE TEMPERATURE INCREASES AND RANGES BETWEEN 180 MINUTES AT 70*C. AND 10 MINUTES AT 120*C., THE TIME OF HEATING BEING INSUFFICIENT TO CAUSE THE ACETYL VALUE OF THE COLLEULOSE TRIACETATE TO FALL BELOW 59%. 